OKT6 monoclonal antibody was used to detect Langerhans cells in bronchoalveolar lavage fluid from 131 patients in order to study the specificity of the finding of OKT6-positive cells for the diagnosis of pulmonary histiocytosis X. OKT6-reactive cells were found in all patients with histiocytosis X (mean plus or minus SEM, 5.29% plus or minus 1.4% of all cells in lavage fluid frm 18 patients). The number of OKT6-positive cells in the other 113 patients was significantly smaller (mean plus or minus SEM, 0.20 plus or minus 0.04% of all cells, p Less than 0.001). However, some overlap was present in a small group of 6 patients who had miscellaneous lung disorders other than pulmonary histiocytosis X and had a positive OKT6 reaction in 1.3 to 2.8% of all cells in lavage fluid. Thus, the finding of ODT6-positive cells in lavage fluid is not absolutely diagnostic of pulmonary histiocytosis X.